In most RF systems, it is common to test cable paths within the system to verify that the cables are capable of signal transmission with minimal degradation. An example of a typical RF cable measurement would be an insertion loss measurement. Insertion loss provides a measure of the power loss along a transmission line and is typically measured in dB. Insertion loss varies with the type of cable, operating frequency, and cable length.
Insertion loss measurements can be used to troubleshoot or to verify cable performance. A common instrument for performing these measurements is a vector network analyzer (VNA). Vector network analyzers can measure and display the complete amplitude and phase characteristics of an electrical network. VNA hardware typically includes a sweeping signal source, a test set to separate forward and reverse signals, and a phase-coherent receiver.
Scattering Parameters, or s-parameters, are the reflection and transmission coefficients between the incident and reflection waves. They are used to describe the behavior of a device under linear conditions at radio frequencies. Each parameter is typically characterized by magnitude, decibel and phase. The expression in decibel is 20log(Sij) because s-parameters are voltage ratios of the waves. For a two port network, S11 is the reflection coefficient of the input, S21 is the forward transmission gain, S12 is the reverse transmission gain, and S22 is the reflection coefficient of the output. S12 and S21 measurements (such as Insertion Loss, Time Delay Matching, and Phase Matching) using a VNA usually involve two test cables. After calibrating out the errors due to the VNA, test cables and adapters, one test cable is connected between a first port on the VNA (Port 1) and one end of the cable under test. A second test cable is connected between a second port of the VNA (Port 2) and the other end of the cable under test. Where the endpoints of the cables or Units Under Test are not physically located close to each other, or the test cables have to be routed around obstructions, longer test cables than desired or practical may be necessary to perform S12 or S21 measurements.
If the length of the test cables significantly exceeds the length of the cables to be tested, the errors introduced by the test cables can begin to cause inaccuracies in the measurements. In many cases, the cables to be tested are installed in equipment, such as an aircraft, and cannot be easily removed.
It would normally be feasible to test short cables by the standard method of attaching a test cable to each side of the cable under test. However, in some instances, such as when testing cables in an aircraft, long test cables would be required so that connections can be made to the ends of the cables under test. Traditional test methods for testing RF cabling that utilize long test cables are expensive due to additional test cable requirements for ruggedness and other cable characteristics. Long test cables have greater insertion loss as well as phase and insertion loss stability problems, and also create potential safety hazards. For example, there is an increased chance of a person tripping over the test cable and causing Foreign Object Debris (FOD) damage or personal injury. Long test cables also result in an increased risk of the test cable being damaged by oil or grease, or being run over or stepped on. In addition, long test cables can be intrusive on other tasks being performed in the immediate area.
There is a need for a cable test method that eliminates the need for long test cables.